As one of the important optical technique in the field of laser sampling, low efficiency beam sampling grating (BSG) is widely used in high power laser system. It is an off axis, binary phase, Fresnel zone plate. In this paper, the principle of the variable period grating′ formation process is analyzed perfectly and the transmissive function of the BSG is obtained. Based on the analysis, a new method is presented to fabricate the BSG with electron beam direct writing. Compared with the holographic method, the new method is simpler, cheaper, and more efficient to manufacture elements on a large scale.